System for reducing interference



Aug. 25, 1942. E. s. PURINGTON I 2,294,129

SYSTEM FOR REDUCING INTERFERENCE Filed Sept. 13, 1940 2 eets-Sheet 1 I,5: INVENTOR E ELL/SON S. PIQVGTON l I BY i I I '-L ATTORNEY A 1942- E.s. PURINGTON 7 2,294,129

SYSTEM FOR REDUCING INTERFERENCE, I

2 sheets-sheet 2 Filed Sept. 13, 1940 vkbu INVENTOR SON SgR/NGTONA1:TORNEY ELL! Patented Aug. 25, 1942 UN ITED STATES P SYSTEM FORREDUCING INTERFERENCE Ellison S. Purington, Gloucester, Mass., assignor,by mesne assignments, to Radio Corporation of America, New York, N. Y.,a corporation of Delaware Application September 13, 1940, Serial No.356,592

7 Claims.

The present invention relates to reduction of interference upon systemsfor superselective transmission and reception employing a plurality ofcontinuous waves or their equivalent.

The system herein described is based upon the use of superaudiblemodulation, in which at the transmitter (1) a radio carrier is modulatedas to amplitude at a superaudible rate, (2) the superaudible rate ischanged in frequency for signalling, (3) the radio carrier is wobbled infrequency for purposes of privacy and for reducing the amount of timethat any interfering transmitter can match the outgoing radiations. Asuitable receiver for this type of transmission includes a radio tuner,first detector to produce current of the modulating frequency, anintermediate or modulating frequency amplifier, a second detector of theheterodyne type to produce audible frequency currents, and usually atuned audio frequency amplifier circuit. Such systems have beendescribed in U. S. Patent #1,68l,293 issued to John Hays Hammond, Jr.,U. S. Patent #1,690,'719, issued to Chaffee et al., and U. S. Patent#1,899,527, issued. to E. S. Purington.

The freedom from interference depends in part upon the wobbler range, bywhich the amount of time that any undesired radiation can disturb thesystem is reduced.

The present invention is concerned primarily with reduction of thetransmission of signals with which the interfering signals would causedisturbance, and also with reduction of the transmission of the receiverat the instant that such interferences would be produced. Either or bothof these devices can be used in order to reduce the interferencepossible on such a system still further than that due to thesuperselective features previously disclosed.

It is to be understood that this invention is applicable to othersystems employing a wobbler at the transmitter for similar purposes, oreven to the transmission and reception of telephonic intelligence by thefrequency modulation process, such as shown in U. S. Patents #1599586 toE. S. Purington and #1,9'7'7,439 to John Hays Hammond, Jr., in which aportion of the signal can be deleted without disturbing the transmissionin which the signal is integrated from.

other portions of the transmission.

The invention also consists in certain novel features of constructionand combinations of parts hereinafter set forth.

Although the novel features which are believed to be characteristic ofthis invention will be particularly pointed out in the claims appendedhereto, the invention itself, as to its objects and advantages, the modeof its operation and the manner of its organization, may be betterunderstood by referring to the following description taken in connectionwith the accompanying drawings forming a part hereof. In said drawings,Fig. 1 illustrates diagrammatically a preferred form of a transmittercircuit incorporating certain of the features of the present invention;and, Fig. 2 is a diagrammatic representation of a receiver which may beused to receive signals transmitted by a transmitter like that shown inFig. 1 and incorporating certain features of the invention.

In Fig. 1, a master carrier oscillator MCO operates through a bufferstage ES to excite a modulated power amplifier MPA, which is modulatedby a modulator M actuated by a modulation oscillator MO. The modulationoscillator MO is provided with a key K1 by means of which the modulationfrequency may be frequency modulated at dot-dash rate for purposes ofsignalling. The master carrier oscillator M00 is frequency modulatedartificially by a Wobbler mechanism WM which may include a motor drivenfan type condenser F, and an operable key K2. Ihe former serves tospread the entire radiation over a band of frequencies, whereas thelatter serves to conceal the changes in side band location due tooperation of the signalling key K1 associated with the modulationoscillator MO.

The transmitted radiations of an arrangement like that disclosedcomprise essentially a carrier wave and an upper and a lower sidefrequency. Whenever any extraneous interference matches either thecarrier or a side band, the receiver response is interfered with. Whilethe use of the motor driven fan type condenser portion of the Wobblerprevents the interference from matching the radiations all of the time,yet the disturbance caused by the interference is not negligible.

For example, if the range of wobble due to the fan is 10,000 cycles, andthe interference is effective when the interfering signal matches anoutging radiation within cycles it can be shown that the interferencemay be effective from 1.4 per cent to 6.4% of the time.

As applied to the transmitter, it is recognized that whenever thereexists an interfering signal which would cause disturbance at thereceiver, the transmitters also will have some of the interfering signalpresent. That is, any transmitter also acts as a receiver. If thedifferences between any continuous wave equivalent of the outgoingradiation for operating the receiver and any incoming continuous waverepresenting interference is for example 1000 cycles, then there will bea 1000 cycle beat note existing in the plate circuit of the modulatedpower amplifier circuit MPA. This is rectified by the power amplifiertube PT, and a current of 1000 cycles is produced, which can be filteredout by an audio filter AF, without disturbing the transmission of energyto the power amplifier for purposes of modulation.

The system may be adjusted so that when the interference is so closethat beat notes of less than 200 cycles say exist, the transmitteroperation may be automatically modified so that beat notes less than 200cycles do not exist at the receiver.

The audio energy in the plate circuit of tube PT for example may befiltered by filter circuit AF, amplified by amplifier A, and rectifiedby rectifier R3, and the rectified output used through switch S1 when inthe position shown to bias the buffer stage tube BT so as to render thebuffer stage BF inoperative or if desired, through switch S2 when in theposition shown to make a sudden change in the carrier frequency so thatthe beat note becomes higher than 200 cycles and increasing, instead oflower than 200 cycles and diminishing. When switch S1 is used in theposition shown, the radiation will be cut off and the system does notcontribute to the signal. This causes the impulse of energy through therectifier R3, to cease, but the time constant of the circuits isarranged to be sufficiently long so that when the system recovers, theaudio beat note is above 200 cycles and increasing. The system can beorganized of course, so that the buffer stage is not completely out off,and the amount of signal merely reduced during the time thatinterference would be produced. In the case where switch S2 is used, thebuffer stage if desired, need not be modified at all in which case theswitch S1 is placed in its upper position. Additional wobbler condensersC1 and C2 controlled by commutators CMI and CMz respectively, are usedso that the frequency of the oscillator Will make a sudden jump. If thelow beat note is produced due to the fan wobbler increasing in capacity,commutators on the wobbler shaft distribute the D. C. output of therectifier through switch S2 so that additional condenser C2 is cut intocircuit, but if the low note results when the fan Wobbler is decreasingin capacity, then the other additional condenser C1 is cut out of thecircuit. The relays R1 and R2 which control the switching of thecondensers are of the quick acting slow release type and return tonormal (the positions shown) when the audio control signal ceases, witha suitable delay such that the fan Wobbler F has advanced far enough tomake a change which is at least equal to the change produced by therelay controlled condensers. It should be understood that in place ofelectromagnetic relays R1 and R2, it is within the scope of theinvention to use electronic frequency shifting devices. However, sincethe rate of wobble for such systems is of the order of 20 wobbles persecond so that several wobbles exist in the duration of a signallingdot, electromagnetic devices of the type shown can be used.

While in general it is preferable to fail to transmit signals at thatportion of the wobble cycle at which interference is produced, it isrecognized that the space relations between the transmitter, receiver,and interfering signal may be such that the receiver may be subject tointerference while the transmitter is screened from the interference. InFig. 2, therefore, there is shown an arrangement for stopping thetransmission of signals through the receiver when there is aninterference so close that disturbances would result. It is to beunderstood that the signal is integrated from all portions of thewobbled cycle, and that the omission of signals for a small portion ofthe cycle will not necessarily be noted. The decrement of the receiverintermediate and audio filters may be sufiiciently low that the signalis practically steady. It can be arranged, of course, that loss ofsignal strength due to the portion of time that the signal is throttledcan be made up by increased gain of the transmission when theinterference does not potentially exist.

In the circuit arrangement of Fig. 2, the usual antenna A, radioampifier RA and first detector D1 are provided as well as a first stageof intermediate amplification IFA1 corresponding to the amplitudemodulation frequency at the transmitter with the key K1 down. In theoutput of this amplifier there will be currents of the signallingfrequency, and also possibly beat notes of changing pitch due tointerferences. For the signal, a second IF amplifier IFAz is providedand this feeds into a circuit including a second detector D2 withheterodyne H, and a tuned audio amplifier TAF. It is clear that when theinterference matches any of the three transmitted radiations, so thatthe IF amplifiers are actuated with the frequencies to which they aretuned, which frequency results in part from the presence of theinterference, then the audio amplifier will be actuated.

Whenever the extraneous energy exists, there will be currents of twofrequencies in the IF input, one associated with the signal (either thekey K1 down or up) and one associated with the interference. Thedifference of the key up and key down frequencies may be of the order ofcycles, and the first IF amplifier not sharp enough to discriminatebetween the key up and key down modulating frequencies.

The subsequent circuits in the signal channel are provided withelectronic gain control circuits, as for example to control the signalgrid of the second detector. A control circuit comprising a controlintermediate frequency amplifier CIFA, is provided for amplifying anddetecting the intermediate frequency energy, whereby audio frequencyenergy of continuously varying pitch in accordance with the transmitterwobble is produced whenever there is potentially an interference uponthe system. The control circuit includes amplifier CIFA and detectorCD1. The audio frequency is filtered by a resistance-condenser typefilter RCF with suitable cut off, so that when the beat note falls belowa desired value, say 300 cycles, a rectifier CD2 is energized whichdevelops a D. C. voltage impulse which is impressed upon the controlgrid of the second detector D2, and blocks the transmission of signals.The circuit recovers when the beat note becomes higher than 300 cycles,and no interference is producable. An additional feature of theinvention comprises a tube ST which shunts the controlled tube D2 and isoppositely actuated whereby the total current through the input to theaudio circuit does not change, thus eliminating shock excitation.

The system here shown is also applicable to reduction of disturbancessuch as upon the two phase signalling system of U. S, application316,428 of John Hays Hammond, Jr., or to any system in which frequencymodulation of a carrier is used to reduce the amount of time thatinterferences can be produced by interfering signals.

Although only a few of the various forms in which this invention may beembodied have been shown herein, it is to be understood that theinvention is not limited to any specific construction but might beembodied in various forms without departing from the spirit of theinvention or the scope of the appended claims.

I claim:

1. In a system of communication of the type wherein the transmitter isfrequency modulated to thereby reduce the amount of time that acontinuous wave transmitter can match the outgoing radiation andharmfully disturb reception, means at the transmitter actingautomatically when such interfering energy is intercepted by thetransmitter for modifying the transmission characteristics of the systemwhereby the transmission is reduced when a small frequency differenceexists between an interferring radiation and a frequency modulatedconstitutent of the radiated energy and said means including an outputstage of the transmitter.

2. In a system of communication in which the carrier frequency energy iswobbled between certain frequencies to thereby reduce the amount of timethat interfering energy of predetermined frequency can match theoutgoing radiation and. thereby harmfully disturb reception, the methodof substantially reducing interference with the transmitted signal whenthe carrier frequency is at such a value that the frequency differencebetween the interfering energy and one of the constituents of theradiated signal energy constitutes a lower order of frequencies, whichcomprises combining at a point in the system prior to radiation of thetransmitted signal the interfering energy and said constituent of theradiated energy, producing from said combination a direct currentcomponent and utilizing said direct current component to substantiallyreduce the amount of transmission for an appreciable period of time tothereby allow the carrier frequency to change substantially from saidvalue.

3. In a system of communication wherein carrier frequency energy iswobbled between certain frequencies to thereby reduce the amount of timethat interfering energy of predetermined frequency can match theoutgoing radiation and thereby harmfully disturb reception, the methodof substantially reducing interference with the transmitted signal whenthe carrier frequency is at such a value that the frequency differentbetween the interfering energy and one of the constituents of theradiated signal energy constitutes an audio frequency which comprisescombining at a point in the system prior to radiation of the transmittedsignal the interfering energy and said constituent of the signal energyand producing therefrom direct current component and utilizing saidproduced direct current component to substantially reduce the amount oftransmission for an appreciable period of time suificient to allow thecarrier frequency to change substantially from said value.

4. In a system of communication wherein carrier frequency energy iswobbled between certain frequencies to thereby reduce the amount of timethat interfering energy of predetermined frequency can match theoutgoing radiation and thereby harmfully disturb reception, the methodof substantially reducing interference with the transmitted signal whenthe carrier frequency is at such a value that the frequency differencebetween the interfering energy and one of the constituents of theradiated signal energy constitutes an audio-frequency, which comprisescombining at a point in the system prior to radiation of the transmittedsignal the interfering energy and said constituent of the signal energyand producing therefrom a direct current component and utilizing saiddirect current component to stop thev transmission of said carrierfrequency for an appreciable period of time sufficient to allow thecarrier frequency to change substantially from said value.

5. In a system of communication wherein the carrier frequency energy iswobbled between certain frequencies to thereby reduce the amount of timethat interfering energy of predetermined frequency can match theoutgoing radiation and thereby harmfully disturb reception, the methodof substantially reducing interference with the transmitted signal whenthe carrier frequency is at such a value that the frequency differencebetween the interfering energy and one of the constituents of theradiated signal energy constitutes an audio frequency which comprisescombining at a point in the system prior to radiation of the transmittedsignal the interfering energy and said constituent of the signal energyand producing therefrom direct current component and utilizing saiddirect current component to shift the frequency of the carrier frequencyenergy to a different value.

6. In a system of communication wherein the carrier frequency energy iswobbled back and forth between certain frequencies to thereby reduce theamount of time that interfering energy of predetermined frequency canmatch the outgoing radiation and thereby harmfully disturb reception,the method of substantially reducing interference with the transmittedsignal when the carrier frequency is at such a value that the frequencydifference between the interfering energy and one of the constituents ofthe radiated signal energy constitutes an audio frequency whichcomprises combining at a point in the system prior to radiation of thetransmitted signal the interfering energy and said constituent of thesignal energy and producing therefrom direct current component andutilizing said produced direct current component to shift the frequencyof the carrier frequency energy upward when the wobbler is operating ina direction which increases the frequency of the carrier frequencyenergy and downward when the wobbler is operating in a direction whichdecreases the frequency of the carrier frequency energy.

'7. In a signalling system of the type wherein carrier energy modulatedin accordance with signal energy desired to be transmitted is radiatedand received at a receiving station, the method of substantiallyreducing interference with the transmitted signal energy when thedifference in frequency between the interfering energy and a componentof the radiated energy constitutes a disturbing frequency whichcomprises combining at a point in the system prior to radiation of thetransmitted signal the interfering energy and said component of theradiated energy and producing from said combination a control energy andutilizing the control energy to change the frequency of the carrierenergy to such a value that said difference ceases to constitute adisturbing frequency.

ELLISON S. PURINGTON.

